D16222 has some extra stuff, but the changes to PhamePostViewController->buildCommentForm() in that change are generally reasonable as a model. Or just look at PhabricatorPasteViewController for a possibly-simpler example. After making this change, delete PhabricatorPhurlURLCommentController and the route for it (these will be fully handled by EditEngine, and no longer need a separate route). This change will enable the "Change Subscribers" and "Change Project Tags" actions when adding a comment, improve the draft code, and reduce the total amount of code in the codebase.